Recently, submarine communication systems using submarine cables are increasingly introducing an optical add-drop multiplexing (OADM) system and a reconfigurable OADM (ROADM) system.
FIG. 6 is a block diagram illustrating a general configuration of a submarine communication system 900 introducing an OADM system or an ROADM system (hereinafter, referred to as an “OADM/ROADM system”). A node 910 includes an OADM/ROADM function.
Sub-bands 1 to 3 indicate wavelength bands of optical signals. An optical signal, which is transmitted from a trunk station 51 and of which wavelength band is sub-band 1, is addressed to a trunk station 52, and the optical signal, of which wavelength band is sub-band 2, is addressed to a branch station 53. It is noted that hereinafter the “optical signal, of which wavelength band is sub-band 1 (or 2, 3)” is referred to as the “optical signal of sub-band 1 (or 2, 3).”
The node 910 transmits the optical signal of sub-band 2, from among the optical signal 13 received from the trunk station 51, to the branch station 53. The node 910 branches the optical signal of sub-band 1 included in the optical signal 13 received from the trunk station 51 into two and respectively transmits the branched optical signals to the trunk station 52 and the branch station 53. Further, the node 910 removes a dummy signal 5 from an optical signal 15 received from the branch station 53. Then, the node 910 transmits an optical signal 14 that combines the optical signal of sub-band 3 that was received from the branch station 53 and the optical signal of sub-band 1 that was received from the trunk station 51 to the trunk station 52. Optical submarine relay devices, not illustrated, are installed on the submarine cables between the node 910 and the trunk stations 51, 52 and between the node 910 and the branch station 53.
In FIG. 6, as the optical signal addressed to the branch station 53 is only the optical signal of sub-band 2, the optical signal of sub-band 1 addressed to the trunk station 52 is not necessarily transmitted to the branch station 53. However, by additionally transmitting the optical signal of sub-band 1 to the branch station 53, the input power to an optical submarine relay device that is installed between the node 910 and the branch station 53 can be maintained without too much drop compared with the optical signal 13. As the result, the optical submarine relay devices used in the submarine communication system 900 can operate within a predetermined rating range that is common throughout the submarine communication system 900. For the same reason, the branch station 53 transmits a dummy signal 5 in addition to the optical signal of sub-band 3.
FIG. 7 is a diagram illustrating wavelength bands of optical signals 13 to 15 that are transmitted to and from the node 910. In FIG. 7, the wavelength band of the optical signal 13 is divided into two wavelength bands, the optical signal 1 of sub-band 1 and the optical signal 2 of sub-band 2. The wavelength band of the optical signal 14 is divided into two wavelength bands, the optical signal 1 of sub-band 1 and the optical signal 3 of sub-band 3. Further, the wavelength band of the optical signal 15 is divided into two wavelength bands, the dummy signal and the optical signal 3 of sub-band 3. The wavelength band of sub-band 1 coincides with the wavelength band of the dummy signal. The wavelength band of sub-band 2 also coincides with the wavelength band of sub-band 3. The wavelength band of sub-band 1 does not overlap the wavelength band of sub-band 2. The optical signals 13 to 15 have channels, each of which can transmit at least one carrier (optical carrier waves). The optical signals of sub-bands 1 to 3 also have channels, each of which can transmit at least one carrier. The carriers are wavelength-multiplexed and transmitted respectively as the optical signals 13 to 15.
FIG. 8 is a diagram illustrating a more detailed configuration of the submarine communication system 900 illustrated in FIG. 6. The submarine communication system 900 includes trunk stations 51 and 52, a branch station 53, and a node 910. The trunk stations 51, 52 and branch station 53 transmit and receive the optical signals 1 to 3 and dummy signal as illustrated in FIG. 7 via the node 910. The node 910 includes optical couplers 6 and 12 and wavelength filters 7 and 8. The trunk stations 51, 52 and the branch station 53 are connected to the node 910 by submarine cables including optical submarine relay devices 54.
The trunk station 51 transmits the optical signal 1 of sub-band 1 and the optical signal 2 of sub-band 2. The optical signals 1, 2 are branched into two at the optical coupler 6 of the node 910. The one of the optical signals 1, 2 branched into two is transmitted to the branch station 53. The other one of the optical signals 1, 2 branched into two at the coupler 6 is transmitted to the trunk station 52 through the wavelength filter 7 and the optical coupler 12. The wavelength filter 7 blocks the optical signal of the wavelength band of sub-band 2.
As described above, the optical signal 1, as well as, the optical signal 2 are transmitted from the node 910 to the branch station 53 in such a way that the optical submarine relay device 54 installed on the transmission path from the node 910 to the branch station 53 operates within a rating range. As the result, the branch station 53 receives the optical signal 2 that is the target signal, as well as, the optical signal 1 that is addressed to the trunk station 52.
In relation to the present invention, PTL 1 discloses a technique of preventing a terminal station from receiving unnecessary optical signals by blocking the unnecessary optical signals in corresponding sub-band using a cut-off filter arranged on a transmission path.